It is possible to get thermals from cities but unlikely to effect a 737 to the point of people making comments. Have just looked at the weather at your time of arrival and wind was gusting from 20-30kts. This is what caused the turbulence you felt. The adding of power is more to maintain airspeed rather than altitude, obviously if you get too slow you will be unable to maintain altitude but the initial response of adding power will always be to maintain airspeed if it is a short momentary burst of power.
Thermals are more noticeable on calm hot days, I assume as stronger winds diffuse the thermal activity.